A portion of the disclosure of this patent document contains material that is subject to copyright protection. The owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.
This invention is related to novel methods and tools for use in manufacturing parts with improved fatigue life, particularly for parts having fastener apertures therein, or cutouts therein, and which parts are subject to repeated or prolonged stress. More specifically, this invention relates to improved techniques for utilizing the stress wave method for working parts, to improved tools for utilizing the stress wave method for working parts, and to finished parts made thereby, which parts have improved stress fatigue resistance characteristics.
Metal fatigue is a problem common to just about everything that experiences cyclic stresses. Such problems are especially important in transportation equipment, such as aircraft, ships, trains, cars, and the like. Metal fatigue can be defined as the progressive damage, usually evidenced in the form of cracks, that occurs to structures as a result of cyclic loading. This failure mode is not to be confused with a failure due to overload. The lower surface of an aircraft wing is a classical example of the type of loading that produces fatigue. The wing is subjected to various cyclic stresses resulting from gust, maneuver, taxi and take-off loads, which over the lifetime of a particular part eventually produces fatigue damage. Similarly, the pressurized envelope of an aircraft, including the fuselage skin and rear pressure bulkhead, are subject to a stress cycle on each flight where the aircraft interior is pressurized.
One problem inherent in fatigue damage is that it can be hidden since it generally occurs under loads that do not result in yielding of the structure. Fatigue damage is most often observed as the initiation and growth of small cracks from areas of highly concentrated stress. Undetected, a crack can grow until it reaches a critical size. At that point, the individual structural member can suddenly fail. Catastrophic failure of an entire structure can also occur when other members of the adjacent portions of the overall structure can not carry the additional load that is not being carried by the failed structural member.
Even stationary objects, such as railroad track or pressure vessels, may fail in fatigue because of cyclic stresses. Cyclic loads for railroad track are caused by repeated loading from the wheels running over an unsupported span of track. In fact, some of the earliest examples of fatigue failures were in the railroad industry and in the bridge building industry. Sudden pressure vessel failures can be caused by fatigue damage that has resulted from repeated pressurization cycles. Importantly, government studies report that fatigue damage is a significant economic factor in the U.S. economy.
Fatigue can be defined as the progressive damage, generally in the form of cracks, that occur in structures due to cyclic loads. Cracks typically occur at apertures (holes), fillets, radii and other changes in structural cross-section, as at such points, stress is concentrated. Additionally, such points often are found to contain small defects from which cracks initiate. Moreover, the simple fact that the discontinuity in a structural member such as a fuselage or wing skin from a hole or cutout forces the load to be carried around the periphery of such hole or cutout. Because of this phenomenon, it is typically found that stress levels in the structure adjacent to fastener holes or cutouts experience stress levels at least three times greater than the nominal stress which would be experienced at such location, absent the hole or cutout.
It is generally recognized in the art that the fatigue life in a structure at the location of a through aperture or cutout can be significantly improved by imparting beneficial residual stresses around such aperture or cutout. Various methods have been heretofore employed to impart beneficial residual stress at such holes or cutouts. Previously known or used methods include roller burnishing, ballizing, and split sleeve cold expansion, split mandrel cold working, shot peening, and pad coining. Generally, the compressive stresses imparted by the just mentioned processes improve fatigue life by reducing the maximum stresses of the applied cyclic loads at the edge of the hole. Collectively, these processes have been generically referred to as cold working. The term cold working is associated with metal forming processes where the process temperature is lower than the recrystallization temperature of the metal. A similar term, xe2x80x9ccold expansionxe2x80x9d, as used by Fatigue Technology Inc., of Tukwila, Wash., is often used interchangeably with cold working, but as applied specifically to their split sleeve cold expansion process. However, of all the methods used to cold work holes, presently the most widely used processes are the split sleeve process and split mandrel process. Together, these processes are referred to as mandrel cold working processes.
Historically, mandrel cold working was accomplished through strictly manual means. As an example, split sleeve cold expansion of holes is still done using hand-held hydraulic tools attached to air-actuated hydraulic power units. The variables involved in tool selection, implementation, and control of the cold expansion process requires skilled operators to reliably produce properly treated holes. Unfortunately, the requirement of having a skilled operator to perform the task is a disadvantage in that it continuously presents the risk of improper or inaccurate processing. Also, such labor-intensive techniques effectively preclude automated feedback necessary for statistical process control. Although development of that process continues, the complexity of the split sleeve processes and the apparatus utilized presently precludes the widespread adoption of the process for automated fastening environments. The split mandrel process it at a similar stage of development; manually performed, but with some minor automation.
The mandrel cold working processes have a particular disadvantage in that they require precision in the size of the starting holes, usually in the range of from about 0.002 inch to about 0.003 inch in diametric tolerance, in order to achieve uniform expansion. Also, an undersize starting hole is required in that process, in order to account for the permanent expansion of the hole and the subsequent final ream that is necessary to remove both the localized surface upset around the periphery of the hole, as well as the axial ridge(s) left behind by the edges of the sleeve split or mandrel splits at their working location within the aperture, and of course, to size the holes. Moreover, treatment requires the use of two reamers; one that is undersized, for the starting hole diameter, and one which is provided at the larger, final hole diameter.
Another undesirable limitation of mandrel cold working processes is the requirement for, presence of, and residual effect of lubricants. For the split sleeve cold expansion process the starting hole must be free of residual lubricants (used for drilling) to prevent sleeve collapse during processing. A collapsed sleeve can be very difficult to remove and necessitates increasing the hole diameter beyond the nominal size, to remove the subsequent damage. The split mandrel process uses a liquid cetyl alcohol lubricant that must be cleaned from the hole after cold working, in order to ensure proper paint adhesion. In either case, the cold worked hole must be cleaned with solvents, in order to remove lubricants. Such chemical solvents are costly, require additional man-hours for handling and disposal, and if not effectively controlled during use or disposal, can have a deleterious effect on operators and/or the environment.
Still another limitation of the prior art mandrel cold working processes is their effect on the surface of the aperture being treated, i.e. the metal wall which defines the hole. The xe2x80x9csplitxe2x80x9d in the split sleeve or the multiple splits in a split mandrel can cause troublesome shear tears in type 7050 aluminum, and in some other alloys. Shear tears, which are small cracks in the structural material near the split(s), are caused by the relative movement of the material near the split. Significantly, the increasing use of type 7050 aluminum in aircraft structures has created a large increase in the number of shear tears reported. Although such tears are generally dismissed as cosmetic flaws, they nevertheless produce false positives in non-destructive inspections for cracks.
Also, in the mandrel cold working processes, the sliding action of a mandrel produces a large amount of surface upsetting around the periphery of the hole, especially on the side of the structure where the mandrel exits the hole. In the split mandrel process, this effect is clearly seen, because of the direct contact of the mandrel with the aperture sidewall. The undesirable surface upset can increase the susceptibility to fretting, which may lead to a reduction in life for fastened joints. Additionally, surface upset in a stackup of structural layers can cause disruption of the sealant in the faying surface. To some extent the undesirable surface upset can be reamed out when sizing the final hole diameter, but at least some portion (and normally a substantial portion) remains.
Pad coining is another process that has been used to improve the fatigue life of holes and other cutouts. This process is described in U.S. Pat. No. 3,796,086 issued Mar. 12, 1974 to Phillips for Ring Pad Stress Coining, and the related, commonly owned U.S. Pat. No. 3,434,327, issued Apr. 16, 1974 to Speakman for Ring Pad Stress Coining Tooling. This method uses opposing dies to cold work an existing hole or aperture. The pad coin process leaves a characteristic concentric impression around the periphery of the cutout. The reduced thickness impression is a major drawback of the process, since the reduced section thickness reduces the bearing area of the hole. Further, the impression makes attaching thin structure at treated fastener holes problematic, since a panel may buckle when the fastener is tightened. Moreover, the process does not attempt to perform ring pad stress coining on a structure prior to machining the hole.
As described in U.S. Pat. No. 3,824,824 issued to Leftheris on Jul. 23, 1974, and entitled Method and Apparatus for Deforming Metal, the stress wave phenomenon has previously been used to deform a metal workpiece by passing stress waves through the workpiece to momentarily render the metal plastic. Such methods and related devices have been employed for metal forming, riveting and spot welding operations.
Another invention by Leftheris, U.S. Pat. No. 4,129,028 issued on Dec. 12, 1978 for a Method and Apparatus for Working a Hole, couples mandrel cold working to the aforementioned stress wave process. The object of this latter mentioned invention was to simultaneously cold work and control the finish and dimensional characteristics of a hole. The process treats both straight and tapered starting holes by driving tapered mandrels through or into an existing hole, using a stress wave generator. The invention teaches production of close tolerance holes to a surface finish of 30 micro-inch RMS. However, as with the other mandrel cold working methods, this process requires a close tolerance starting hole, and is subject to the same surface upset problem as the other mandrel cold working methods. Thus, while this variation of Leftheris""s work realized that it would be advantageous to utilize stress waves to impart residual stresses in structures in an amount sufficient to provide improved fatigue life, the process still suffers from the same starting hole methodology that is used with the mandrel cold working processes.
Another attempt to provide a method for cold working holes was developed by Wong and Rajic, as taught in WIPO International Publication Number WO 93/09890, published May 27, 1993, entitled Improving Fatigue Life of Holes. The method was an improvement over the pad coining methods, because the impression made in the structure being treated is smaller than the hole diameter, thus eliminating the undesirable concentric ring provided in coining methods. Also, although such teaching was advantageous in that it eliminated the need for preparing the starting hole that is required with the mandrel and coining processes, a significant drawback to the Wong process was that it required relatively high loads to indent or cold work the structure being treated. This can be understood from considering minimum quasi-static mandrel load necessary to initially indent a sheet. The initial mean contact pressure, PM, for initial yield (indentation) is estimated by the following equation:
PM=1.10xc3x97(compressive yield stress)
The load P for initial yielding or indenting is calculated by multiplying PM, by the cross sectional area of the mandrel. Therefore:
Mandrel Load (P)=1.10 (compressive yield stress) (mandrel cross sectional area) In practicality, the load necessary to impart fatigue improvement is far greater. For example, the 0.063 inch (1.6 mm) thick 2024-T3 aluminum specimens used in the Wong/Rajic disclosure were cold worked with a (0.158 inch diameter) 4.0 mm diameter cylindrical mandrel. The initial mandrel indentation load using these parameters is calculated at 835 pounds (3714 Newtons). Because the indentation process must go well beyond the initial indentation load to achieve fatigue life improvement, the force used in the Wong/Rajic test ranged from 3595 pounds (15991 N) to 4045 pounds (17994 N) for the (0.158 in.) 4.0 mm diameter mandrel. As a comparison, the forces necessary to cold work (indent) a common xc2xc inch (6.35 mm) diameter fastener can be as high as 10,000 pounds (44484 N). Unfortunately, loads of such magnitude generally require large and bulky machinery such as power presses, hydraulic presses, drop hammers, etc., and as a result, their use is precluded from widespread use in automated fastening systems.
The impracticality of such just mentioned heavy, large equipment for automated fastening are identified by Zieve in U.S. Pat. No. 4,862,043. Commenting on the prior art apparatus, Zieve states, xe2x80x9c . . . a C-yoke squeezer is a large, expensive device which extends around the workpiece to provide an integral backing member. However, such devices are impractical for many applications, since the throat depth requirements, i.e., the distance of the rivet from the edge of the workpiece, result in an apparatus which is impractically large and expensive because of the corresponding stiffness demanded for the required throat depth.xe2x80x9d It is clear that the Wong/Rajic invention dos not teach the propagation of stress waves into the metal for deformation and subsequent residual stress development. Therefore, they do not anticipate the use of stress wave technology to significantly lower the strength and size requirements of the processing device or its supporting structure.
The mandrels in the Wong/Rajic disclosure are designed for the purposes of both indenting and hole punching. While their invention allows for mandrel end shapes to be flat or conical, they do not use the shape of the mandrel end to optimize the extent of the residual stresses. A large and uniform zone of residual stresses is required to produce the highest fatigue life. A mandrel that has a flat end is well suited for forming or punching the hole, but induces a low amount of residual stress at the surface of the sheet. On the other hand, mandrels that have a conical end increase surface residual stresses but tend to xe2x80x9cplowxe2x80x9d the material radially and produce substantial surface upsetting. It is clear then that the prior art, in regards to the configuration of the mandrel ends, does not optimize the extent and depth of the residual stresses.
The Wong/Rajic process can also be used to treat non-circular cutouts using either of two methods. The first method uses a solid mandrel with the same cross sectional shape of the hole. The second method treats selected areas of the cutout using solid circular mandrels prior to machining the cutout. The second method is similar to the invention of Landy, U.S. Pat. No. 4,885,829 which uses the split sleeve cold expansion process to treat selected radii of the cutout. After machining the cutout sufficient residual stresses remain in the radii to improve fatigue life. Another invention by Easterbrook and Landy, U.S. Pat. No. 4,934,170, treats existing non-circular holes and cutouts using tools that conform to the shape of the hole. A common weakness of each of these methods are that only selected areas (radii) of the cutout are cold expanded. The non-uniformity of the residual stresses caused by treating only the radii of the cutout allows for tensile stresses to be present at the hole edge. This has the potential to reduce fatigue life.
The aforementioned invention by Zieve, and others similar to it, are used to drive rivets and fasteners using electromagnetic drivers. Such techniques and apparatuses, however, are not used for cold working a metal structure prior to machining the hole. Hence, in summary, present methods of cold working holes and other cutouts using tapered mandrel methods, coining, punching, and such are not adaptable to automated fastening systems and other automated environments because of their complexity and bulkiness of equipment. Also, presently known methods used by others do not treat the entire periphery of non-circular cutouts leading to potential fatigue life degradation. Finally, prior art countersink cold working methods require re-machining of the formed countersink to achieve the desired fastener flushness.
Thus, the following items summarize the shortcomings of the current methods and will be used as a basis for comparing with my novel, improved stress wave fabrication method. Heretofore known processes are not entirely satisfactory because:
They often require mandrels, split or solid, and disposable split sleeves, which demand precision dimensions, which make them costly;
Mandrels and sleeves are an inventory and handling item that increases actual manufacturing costs when they are employed;
xe2x80x9cMandrelxe2x80x9d methods require a different mandrel for roughly each 0.003 to 0.005 inch change in hole diameter, since each sleeve is matched to a particular mandrel diameter, and consequently, the mandrel system does not have the flexibility to do a wide range of hole existing hole diameters;
Each hole diameter processed with xe2x80x9cmandrelxe2x80x9d methods requires two sets of reamers to finish the hole, one for the starting dimension and another for the final dimension;
Mandrel methods rely on tooling and hole dimensions to control the amount of residual stress in the part, and therefore the applied expansion can be varied only with a change of tooling;
Mandrel methods require some sort of lubricant; such lubricants, and especially the liquids, require solvent clean up;
Splits in a sleeve or splits in a mandrel can cause troublesome shear tears in certain 7000 series aluminum alloys;
The pulling action against mandrels, coupled with the aperture expansion achieved in the process, produces large surface marring and upsets around the periphery of the aperture;
Split sleeve methods are not easily adapted to the requirements of automation, since the cycle time is rather long when compared with the currently employed automated riveting equipment;
Mandrel methods are generally too expensive to be applied to many critical structures such as to aircraft fuselage joints, and to large non-circular cutouts;
Mandrel methods have limited quality control/quality assurance process control, as usually inspections are limited to physical measurements by a trained operator.
My novel stress wave manufacturing process can be advantageously applied to apertures for fasteners, to large holes, to non-round cutouts from a workpiece, and to other structural configurations. Treating a workpiece structure for fatigue life improvement, prior to fabricating the aperture itself, has significant technical and cost advantages. The method is simple, easily applied to robotic manufacturing methods, and is otherwise superior to those manufacturing methods heretofore used or proposed.
From the foregoing, it will be apparent to the reader that one important and primary object of the present invention resides in the use of a novel method for treating a workpiece to reduce fatigue stress degradation of the part while in service, and to novel tool shapes for achieving such results. The method and novel tools simplify the manufacturing costs, and importantly, simplifies and improves quality control in the manufacture of parts having an improved fatigue life.
Other important but more specific objects of the invention reside in the provision of an improved manufacturing process for enhanced service life metal parts subject to fatigue stress, as described herein, which:
eliminates the requirement for purchase, storage, and maintenance of mandrels;
eliminates the requirement for purchase, storage, and maintenance of split sleeves;
eliminates the need for disposal of split sleeves;
eliminates the need for lubrication and subsequent clean-up during manufacture of structures containing apertures therethrough;
enables the manufacture, of a wide range of aperture diameters, in which appropriate fastener diameters can be employed;
allows the magnitude and depth of the residual stresses to be carefully controlled, by way of the amount of energy input into the stress wave;
enables process control to be established using stastical feedback into the manufacturing system, thus enhancing quality assurance;
eliminates shear tears in a workpiece that are commonly encountered in mandrel manufacturing methods;
significantly reduces or effectively eliminates surface marring and upset associated with mandrel methods, thus significantly increasing fatigue life;
is readily adaptable to automated manufacturing equipment, since manufacturing cycle times are roughly equivalent to, or less than, cycle times for automated riveting operations;
eliminates bulky hydraulic manufacturing equipment typically used in mandrel methods, and substitutes simple preferably electromagnetic equipment;
enables aperture creation after fatigue treatment, by a single reaming operation, rather than with two reaming operations as has been commonly practiced heretofore;
is sufficiently low in cost that it can be cost effectively applied to a number of critical structures, including fuselage structures.
Other important objects, features, and additional advantages of my invention will become apparent to the reader from the foregoing and from the appended claims and the ensuing detailed description, as the discussion below proceeds in conjunction with examination of the accompanying drawing.
I have now invented, and disclose herein, an improved cold metal working process that uses stress waves to impart beneficial residual stresses to holes and other features in parts subject to strength degradation through stress fatigue. This improved stress wave process does not have the above-discussed drawbacks common to heretofore utilized cold working methods of which I am aware. The process overcomes the heretofore-encountered shortcomings of cold working processes. Also, it eliminates undesirable equipment necessary for the more commonly utilized alternative processes, such as the need for bulky hydraulic equipment, precision mandrels, disposable split sleeves and messy lubricants. Thus, it is believed that my novel method will substantially reduce manufacturing costs. In addition, my stress wave process is readily adaptable to use in automated manufacturing equipment. As a result, the unique process described herein is a major improvement over other processes in common use today, including mandrel processes.
My improved stress wave method imparts beneficial stresses using a dynamic indenter that impinges the surface of the metal, preferably in a normal direction to the surface. The action of the probe causes waves of elastic and plastic stress to develop and propagate through the metal. In some cases a stationary indenter is used to support thin workpiece materials. Such xe2x80x9cbackingxe2x80x9d indenters also assist in the reflection and or creation of plastic wave of or from the other side of the workpiece.
After a properly applied and focused plastic stress wave has imparted a large zone of residual stress, the area is now ready for the hole. A drill, reamer or other cutting device is positioned concentric to the impact zone from the probe and anvil. When the hole is machined a small rebound of the stresses surrounding the hole occurs. Such rebound manifests itself as shrinking of the machined hole. For this reason, the cutting tools used in my stress wave method may require the use of a feature that takes into account the inward metal movement in a hole. Otherwise, the workpiece material has the possibility of binding on the cutting tool. This could lead to short tool life or poor hole finish. For a drill or reamer, a simple solution to this requirement is to provide a back-taper feature. As a result, substantially uniform beneficial residual compressive stresses remain in the finished structure.